Influence of climate and floristic diversity on the phenology of freely assembled plant communities

A) Phenological studies have so far considered abiotic factors as the main drivers of the growing season length of plant communities. Additionally, plant species diversity may influence the growing season of vegetation stands according to two main, non-exclusive, developmental scenarios: functional complementarity among species (the complementarity effect) and functional selection (the selection effect). The objective of this study was to evaluate if plant species richness can prolong the active photosynthetic period length (APPL) of freely assembled riparian herbaceous communities in agroecosystems of Eastern Canada. We selected a total of 44 herbaceous communities in riparian buffers of varying species richness. All riparian buffers were comparable to one another in their clay content, water table depth and degree-day values. Independently of abiotic factors such as ground water and air temperature, we observed a general positive relationship between species richness and APPL of riparian herbaceous communities. In 2012, the APPL of species-rich communities was, on average, 30 days longer than the APPL of species-poor communities. Species-rich communities were sheltering herbaceous species with delayed green offset dates in comparison to communities dominated by Phalaris arundinacea, a competitive and dominant species in these riparian buffers. This relationship was not observed in 2013, probably because the summer of2013 was within climate averages in comparison to the exceptionally dry climatic conditions of 2012. We estimated that restoring the species diversity and composition of riparian herbaceous communities in intensively managed agroecosystems could account for a net carbon uptake gain of up to 900 kg C yr-1 ha-1. This effect may be of particular relevance in an era of ongoing species extirpation and community homogenization.